Xi Jinping (centre) chats with children at China Agricultural University in Beijing after an unexplained two-week absence. Photograph: Lan Hongguang/Xinhua Press/Corbis

Chinese leader-in-waiting Xi Jinping has reappeared in public and made an impromptu speech following a two-week absence that had sparked rumours about his health and raised questions about the stability of the country's succession process.

State media said Xi toured exhibits at China Agricultural University in Beijing commemorating National Science Popularisation Day, but offered no explanation of why he had dropped from sight.

Photographs posted on the government's official website showed Xi walking in the sunshine dressed casually in an open-necked shirt and black coat. Another photo showed him smiling as he looked at potted plants, showing no sign of disability or ill health.

A lengthy Chinese-language report from the official Xinhua news agency did not address why Xi had not been seen publicly since 1 September, when he made a speech at the ruling Communist party's official training academy.

Since then he has cancelled meetings with visiting foreign dignitaries including US secretary of state Hillary Clinton, Singapore prime minister Lee Hsien Loong and Danish PM Helle Thorning-Schmidt. The Chinese government has yet to explain Xi's public absence.

Speculation over Xi's absence highlights the intense scrutiny China's succession process is under, tempered with uneasiness about the country's opaque political system, which often seems at odds with its rising global importance.

"The leadership needs to realise how the world perceives this. They may have their own reasons for keeping secret, but it is not beneficial to China's global status and position as a world power," said David Zweig, an expert on Chinese politics at Hong Kong University of Science and Technology.

Xinhua said Xi, while visiting the university, spoke about food safety and made an impromptu speech in which he praised the university for sharing science with the public.

Xi is due to take over as head of the Communist party at a leadership congress later this year, the first step in a generational power transition that will see him assume the presidency next spring, embarking on what is expected to be a decade at the helm of the world's most populous nation and second largest economy.

In addition to deciding personnel matters, Xi is heavily involved in drafting a major report to be delivered at the congress, as well as possible amendments to the party's constitution. While Xi hasn't indicated what if any changes he plans to make, expectations are high for gradual economic and political reforms to meet China's changing circumstances, three decades after its abandonment of orthodox Marxism.

Xi's absence also came amid the biggest crisis in years in relations with Japan, sparked by a renewed dispute over a group of uninhabited islands in the East China Sea. Amid a wave of anti-Japanese demonstrations around the country, Beijing has taken an unusually hardline stance over the long-running dispute, sending maritime surveillance vessels into what Japan says are its territorial waters around the islands on Friday in a show of resolve.

While Xi is generally considered a political moderate, he comes from a family of stalwart communists and is seen by some as likely to be relatively tough on matters of sovereignty and national dignity.

"It's a critical political time when the whole world is looking at this guy. If they're worried about uncertainty and instability, well … this will just feed the instability," Zweig said.

Early rumours about his public absence said the 59-year-old Xi had thrown his back out swimming or pulled a muscle playing football. As the days passed and Xi was still not seen, speculation escalated to more serious conditions, including a heart attack, stroke and emergency surgery.

While the Communist party has become more sensitive to public opinion over nationalism and social unrest, it reverts to its roots as a clandestine organisation when it comes to leaders' private lives, particularly their health.

The uncertainty surrounding Xi has been heightened by the party's silence on the dates for the party congress, widely expected to be held in late October.

The leader-in-waiting's sudden disappearance on the eve of his ascension also came during a year full of unforeseen and unsettling political developments that had already threatened hopes for a smooth party leadership.

Most notably, the case of Bo Xilai, one of China's most charismatic and ambitious politicians who fell from power in March, remains unsettled.

Bo's former aide, Wang Lijun, is to go on trial on Tuesday in southwest China's Chengdu city. He faces defection, bribery and other charges.

Wang served as the police chief in the city of Chongqing under Bo but lost his job for still unexplained reasons. In February, Wang fled to the US Consulate in Chengdu, where he told U.S. diplomats about his suspicions linking Bo's wife to the murder of a British businessman.

Since then, Bo's wife has been convicted of murdering the Briton, and Bo is under internal party investigation for severe but unspecified disciplinary violations.

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